喜马拉雅造山带的地壳上地幔结构——近震反射观测结果
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摘要
2002—2005年完成了穿越喜马拉雅山的宽频地震探测,采用了沿剖面紧密排列的台站布置并获得了近震中的反射波资料,这对地壳和岩石圈内部速度界面的研究有重要意义。地震数据及研究结果表明,青藏高原的地震主要发生于上地壳范围内,高原莫霍面反射波的纵波 PmP 与横波 SmS 波至清晰可辨且有很强的能量,在跨越雅鲁藏布江时 PmP 和 SmS 均发生了错动,表现为北深南浅。在改则至鲁谷沿线所记录到的发生在拉萨地块的 Fw21事件的地震数据中,拉萨地块内部的莫霍面反射波,尤其是 SmS 波至非常清晰连续,地壳纵波平均速度为 V_p=6.3km/s,表明拉萨地块内部 Moho 面平坦、连续,不存在突起与错位。在此 Moho 面之前出现壳内较强反射界面,埋深45km 可能正是下地壳的顶界面。P208事件中 PmP 和 SmS 均清晰可见,然而在距震中450km 及其以远地段存在两个清晰的震相 P_(LT)及 S_(LT),并且其能量很强,可能是位于160km 深处的岩石圈底界面的反映。根据对近震资料的分析研究,本文建立了该地区的地壳上地幔的结构模型。
The broadband seismic observation through Himalaya was started in 2002 and ended in 2005.The stations are deployed closely along the profile and the reflection wave data of local earthquakes was collected.There is an important significance for studying velocity interfaces inside the crust and upper mantle.The data show that the most earthquakes in Qinghai-Tibet plateau happened in the upper crust.The ProP-arrivals and SmS-arrivals of the reflections from Moho in plateau are very clear with strong energy.The offsets happen in both of Prop and SmS across the Yalu-Tsangpo River,indicating the Moho in the North is deeper than that in the South.The reflection from Moho in the Event Fw21 in Lhasa block was recorded along the profile from Gaize to Lugu with an average P-velocity velocity of 6.3km/s,especially the SmS is very clear and continuous.This case indicates that the Moho within the Lhasa block is much flat and continuous without horizontal relief and vertical offset.Another strong reflection before the Moho reflection is from the interfaceat depth of 45km,which may be the top of lower crust.In the Event P208,both the PmP and SmS are clearly identified. However,Another two strong reflections,P_(LT),S_(LT),respectively exist beyond distance of 450km,and can be deduced to be the reflections from the bottom of the lithosphere at the depth of 160km in this region.Based upon the analysis of local earthquake data,the crust and upper mantle structure across Himalaya orogenic beltis established.
引文
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